Astronomer Fred L. Whipple, 97, who died of an undisclosed ailment Aug. 30 at a hospital near Boston, was a leading sky watcher of the last century. His likening of comets to large, dirty snowballs made of ice and rock ultimately proved correct.

Long based at Harvard University, from 1955 to 1973 he led what is now the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. Earlier, he helped devise a crafty method of confusing enemy radar during World War II and technology to protect spacecraft from collisions with meteors.

His impact ranged from the highly technical to the broadly popular.

For his observatory, he initiated a new design for a giant research telescope. He recommended a simpler mount and multiple mirrors that reduced the weight and cost while allowing for a larger aperture to see more sky.

With two close friends, rocket scientists Willy Ley and Wernher von Braun, he was a science consultant to Collier's magazine, for articles about space exploration, as well as to Disney Studios for its "Conquest of Space" movie in 1955 and a Viking Press book series.

When Dr. Whipple entered the field in the early 1930s, comets were variously believed to be loose collections of dust held together by gravity; the discharge from volcanic eruptions on distant planets; or the product of mysterious phenomena that might spew from the excited mind of a pulp science fiction writer.

Comets long flummoxed scientists. Some of the celestial bodies gained energy and increased their orbits as they streaked through the firmament, while others lost energy and traveled significantly shorter orbits.

Dr. Whipple's "icy comet" model was novel and defied the commonly held assumption that comets were "flying sand banks" or "interplanetary gravel banks, not discrete bodies."

To unravel their behavior, he devised an ingenious theory. Comets, he said, had a frozen nucleus. As the comet approached the sun, its nucleus heated up and caused a propulsive jet of gas and dust to emit. The effect, he said, was like a rocket engine on a spacecraft. The comet's velocity depended on whether the gas was coming out the front, back or side.

In 1950, when he published his ideas, the media dubbed his comets "dirty snowballs," a description on their composition -- frozen water, ammonia, carbon dioxide, methane and dust. A leading model for explaining comets and their origins, his theory was confirmed by the 1986 return of Halley's comet.

A delighted Dr. Whipple said that measurements and photographs taken by the European Space Agency's Giotto spacecraft showed that the comet had a "discrete nucleus about the size of Manhattan."

Fred Lawrence Whipple, the son of a farmer, was born Nov. 5, 1906, in Red Oak, Iowa. An early bout with polio ended his ambition of being a professional tennis player.

Raised in Southern California, he majored in math at the University of California at Los Angeles because the subject was easy for him. He later wrote with some dismay that he never made the tennis team.

A class in astronomy turned him in that direction, and he received a doctorate in that subject in 1931 from the University of California at Berkeley. While in graduate school, he helped map the orbit of the newly discovered planet Pluto.

Joining the Harvard staff in 1931, he and a colleague were soon working with newly available camera technology to study the speed of meteors and the density of the atmosphere.

He was unsatisfied with the contemporaneous knowledge about comets. Analyses of the comet's spectra showed molecules of water, ammonia and methane, and he surmised that the comet heads must contain gases in frozen form in addition to the solid matter seen in meteors. That led to his work on the "dirty snowball" theory.

During World War II, he worked for the U.S. military to develop and produce "confusion reflectors." They were fragments of aluminum foil dropped from Allied aircraft to create a snowfall of images on the Germans' radar that made it impossible to know where the real planes were.

In 1946, 15 years before the first manned spaceflight, Dr. Whipple proposed a technology for spacecraft that still works today to protect them from being whacked by meteors. Known as the meteor bumper, or "Whipple shield," the device is a thin outer skin of metal that causes meteors to disintegrate upon impact, letting only vapor hit the spacecraft.

Dr. Whipple was a key voice during the race to build artificial satellites to coincide with the International Geophysical Year of 1957 to 1958. He created a network of more than 200 volunteer amateurs as well as scientists at professionally staffed tracking stations to monitor the satellites. It was considered the most accurate optical system for collecting satellite data.

"When the Russians won the IGY satellite contest by launching Sputnik in October 1957, the U.S. military refused to release information about it," Dr. Whipple wrote in Science magazine in 2000. "The amateur teams were the only American source of information about Sputnik, easily visible to the naked eye as it circled the world."

For that optical tracking system, President John F. Kennedy presented Dr. Whipple in 1963 with the President's Award for Distinguished Federal Civilian Service, the highest civilian honor for a career federal employee.

Dr. Whipple wrote a standard text in his field, "'Earth, Moon and Planets" (1941), and was credited with discovering six comets. An observatory in Arizona and an asteroid were named in his honor.

In his spare time, he cultivated roses.

His marriage to Dorothy Woods Whipple ended in divorce.

Survivors include his wife, Babette Samuelson Whipple, whom he married in 1946, of Belmont, Mass.; a son from his first marriage; and two daughters from his second marriage.

Fred L. Whipple, shown in a 1957 photo, demonstrates the flight of a small artificial moon launched by Russia.